The field of this invention is inertial instruments, and more particularly, laser gyros.
The Sagnac effect is well-known to define a linear relationship between the rate of rotation of a circuital waveguide, or loop, and the difference in frequency in oppositely directed electromagnetic wave disturbances travelling through that waveguide at resonance. For a circular waveguide, at resonance, this frequency difference .DELTA.f is related to the rotational rate .OMEGA. normal to the plane of the waveguide loop in accordance with: EQU f=(4A/n.lambda.P).OMEGA.
where A is the area of the loop, P is the perimeter of the loop, n is the refractive index of the material of the loop and .lambda. is the free-space wavelength of the wave.
In the prior art, ring lasers have used this principle to provide measurements of inertial rotation in active laser gyros. In such configurations, a ring laser generates two counter-propagating optical signals in a common planar cavity, generally established by three or more mirrors positioned about the laser. When the cavity is rotated with respect to inertial space perpendicular to the plane of the cavity, the effective path length for the two optical signals changes so that one signal encounters a relatively long effective optical path length, and the oppositely-directed signal encounters a relatively short effective optical path length. As the cavity rotates, the beats of the frequency difference for the counter-rotating signals provide a measure of the rate. However, such active laser gyros are affected by the phenomenon of lock-in of the two oppositely-directed optical signals at low rotation rates. In addition, substantial bias drift and scale factor variation are present due to the characteristics of the gain medium (i.e. the laser) within the ring cavity.
Passive laser gyros are also known in the art which avoid the lock-in problem encountered by active laser gyros. The conventional passive laser gyro includes a cavity for the counter-propagating optical signals made up of a plurality of reflecting surfaces or mirrors, with no active gain medium in that closed path. The counter-propagating optical signals are established by a pair of lasers external to the cavity. These signals are generally coupled into the cavity through partly-transmitting mirrors. Servo-loops may be used to vary the frequency of the two lasers to establish resonance in both directions in the cavity.
While this form of gyro avoids the lock-in problem encountered by the active laser gyro configurations, performance is still limited due to the fact that with two lasers, there are two sources of beam, or optical signal, instability. In addition, with the mirrored arrangement for establishing the cavity, the counter-rotating optical signals generally do not have precisely the same propagation paths in the cavity due to alignment errors in the mirrors which project the signals into the cavity.
U.S. Pat. No. 4,135,822 illustrates another form of passive ring resonator laser gyro known in the prior art. This form is similar to the above-described passive configuration, but uses a single laser and a beam splitter to provide the two counter-rotating optical signals. In this form, a pair of servo-loops are used in conjunction with frequency shifters (such as Bragg cells) for each portion of the split beam from the laser. The frequency shifters control the frequencies of the counter-rotating optical signals to achieve resonance during rotation. By way of example, the servo loop for one signal might assure resonance for one of the optical signals by varying the position of one of the mirrors in the cavity by means of a piezoelectric crystal, while maintaining a constant Bragg cell drive frequency for that signal. The other servo loop might utilize a Bragg cell to control the frequency of the second optical signal to achieve resonance.
In this form, a single laser is thus used to avoid the multiple source of instability established by two lasers in the previous passive laser gyro configurations. In addition, the lock-in problem is also avoided. However, in this configuration where the basic cavity is established by mirrors, it is not possible to have both counter rotating optical signals use precisely the same optical propagation path. In U.S. Pat. No. 4,135,822, it is suggested that a single optical fiber might alternatively be used to establish the cavity. While this approach would solve the problem of different optical paths, the losses encountered in joining the two ends of the fiber to establish the circuital cavity causes a serious degradation in performance. Further degradation is introduced by the requirement for coupling the optical signals to such optical fibers using currently known techniques.
Accordingly, it is an object of the present invention to provide an improved passive ring resonator laser gyro.
It is another object to provide an improved passive ring resonator laser gyro having a substantially common path for the counter-rotating optical signals.